The present invention is related to a transmission system comprising a transmitter for transmitting digital symbols via a transmission medium to a receiver, said receiver being provided with a decoder for deriving decoded symbols from an input signal of the receiver having a predetermined signal property out of a plurality of possible signal properties, said receiver comprising property determining means for determining said predetermined signal property from the received signal by determining the reliability of the decoded symbols, the property determining means further being arranged for deriving at least a first sequence of decoded digital symbols assuming a first signal property and a second sequence of decoded digital symbols assuming a second signal property.
The invention is also related to a transmission method and a receiver for receiving a signal having a predetermined signal property out of a plurality of possible signal properties.
A transmission system according to the preamble is known from the paper "Syndrome-based Viterbi decoder Node Synchronisation and Out-of lock Detection" by M. Moeneclay and P. Sanders published in the conference records of GLOBECOM '90, Volume 1, pp. 604-608.
DVB (Digital Video Broadcast) is the new European standard for digital video transmission. This standard prescribes the source coding scheme and the channel coding scheme. For satellite based transmission (DVB-S), the source coding involves a concatenated coding scheme using a Reed-Solomon code as outer code, and a rate 1/2, k=7 convolutional code as inner code. The rate of said convolutional code can be increased by means of puncturing. Between the RS-encoder and the convolutional encoder an interleaver is introduced for spreading burst errors over different Reed-Solomon codewords.
Because a DVB signal can carry various types of video- and audio signals, such a DVB signal can have a large number of signal properties. These signal properties are not known beforehand at the receiver. Signal properties are e.g. a reference phase of the carrier phase in the case of QPSK modulation, the actual rate of the convolutional code used and the starting position of the puncturing map. In case of a non-DVB system in which a block code is used as inner code, the position of the boundary between two codewords is a further signal property. In order to determine the actual signal properties of the received signal, the receiver comprises property determining means for determining said signal properties from the received signal.
In order to find the used signal properties, all possibilities are tried until the correct signal property is found. There are several known way to determine whether an actually tried signal property is the correct one.
A first way to determine whether an actually tried signal property is the correct one is to recode the decoded symbols according to the assumed signal property, and comparing the input symbols with the recoded symbols. If the symbol error rate is small, it is likely that the chosen property is correct. If the error rate is high the chosen property is incorrect, and the next one has to be tried. A disadvantage of the above mentioned method is the required additional implementation complexity needed for the recoding.
A second way to determine whether an actually tried signal property is the correct one is to use a reliability measure for the decoded digital symbols which reliability measure is often already available within the receiver. In the transmission system according to the above mentioned conference paper, the reliability measure used is the path metric growth in the Viterbi decoder for decoding the inner code. In said transmission system, a first signal property is tried, and the reliability measure is compared with a threshold. Subsequently a second signal property is tried, and again the reliability measure is compared with the threshold. When in both cases the reliability measure exceeds the threshold, it is decided that both signal properties were incorrect.
A problem with the above mentioned transmission system is that the reliability measure varies in dependence on the signal property of the transmitted signal. The reliability measure varies strongly with the rate of the inner code. Simulations have shown that the reliability measure for a decoder that incorrectly assumes a rate 3/8 suggests a higher reliability than the reliability measure for a decoder that correctly assumes a rate 1/2. Furthermore the reliability measure is also affected by the signal to noise ratio. This all requires that the threshold levels are set to values that are dependent on the rate of the inner code, the signal to noise ratio etc.